Wing Commander Marsh (Michael Bryant), an assistant to the British Medical Officer, decides to use his extensive knowledge of mental illness for an escape. He proposes to "go insane" and be repatriated. Colonel Preston agrees to let him, so long as he follows through with it to the bitter end. Marsh does a very thorough job: his bizarre, disruptive behaviour continually annoys the other allied officers, who are mostly unaware of the scheme. However, the Germans are not convinced, and Ulmann asks a Corporal to observe Marsh closely. The Corporal has a brother who is insane, so Ulmann believes he is a better judge of Marsh's condition than any doctor. The Kommandant initially refuses to allow the Swiss authority to examine Marsh but relents when Marsh's evident madness embarrasses him in front of an important visitor. By the time the Germans are willing to consider repatriation, Marsh has done such a convincing job that even the Doctor is uncertain whether or not Marsh is simply pretending to be insane. After Marsh has been successfully repatriated to the UK, Colonel Preston receives a letter from Marsh's wife, revealing her husband's feigned psychosis has become genuine, and that he has been committed to a mental hospital for long-term care, with little hope of recovery. Colonel Preston immediately forbids any further escape attempts along the same lines.

The method of escape is based on that used by Ion Ferguson, a Royal Army Medical Corps doctor imprisoned in Colditz, who certified a number of prisoners as insane in Stalag IV-D, who were then repatriated to Britain. Ferguson then feigned his own insanity to gain repatriation in 1945. Ferguson detailed his escape in his account of his wartime experiences, Doctor at War, and the episode, Tweedledum, is a fictionalised account of his means of escape retold as tragedy.